1. Field of the Invention
The invention relates generally to the field of scintillation-type radiation detectors for use in well logging instruments. More specifically, the invention relates to various forms of radiation shielding for such detectors to reduce detection of radiation other than the specific radiation events intended to be detected.
2. Background Art
Various well logging instruments are known in the art that measure radiation phenomena originating from subsurface formations surrounding a wellbore. Such instruments may be inserted into the wellbore at the end of an armored electrical cable, on a pipe string or coiled tubing or other conveyance. Some forms of such instrument include a source of high energy neutrons. The source is configured to emit the neutrons into the formations surrounding the wellbore. The source may be a “chemical” source that includes a radioisotope, for example, americium-241 powder mixed with beryllium powder. Other sources are electrically controlled accelerators, such as deuterium-tritium or deuterium-deuterium accelerators that emit a continuous stream of neutrons or controlled duration “bursts” of neutrons into the formations.
Such well logging instruments include one or more radiation detectors, typically scintillation counters. A scintillation counter includes a crystal made from a material that is sensitive to radiation entering therein. Such material emits a small flash of visible, infrared or ultraviolet light upon interaction with radiation. Typically the amplitude of the flash is related to the energy of the deposited radiation. A converter and amplifier, such as a photomultiplier, is optically coupled to the crystal, and is arranged to generate a detectable electrical pulse corresponding to each radiation detection event. U.S. Pat. No. 7,084,403 issued to Srivastava et al. describes a variety of different materials used for scintillation detector crystals, including their respective advantages and disadvantages.
One particularly useful type of well logging instrument is known under the trademark RST which is a trademark of the assignee of the present invention. Such instruments and its more recent improved implementations thereof, include an accelerator type source of neutrons that emits controlled duration bursts of high energy neutrons into the formations surrounding a wellbore. One or more scintillation type radiation detectors are arranged in the instrument to detect gamma rays resulting from interactions of the neutrons with the surrounding formations. In order for such instruments to provide measurements that are closely representative of the properties of the surrounding formations, it is desirable to shield the one or more radiation detectors from both direct emission of neutrons from the source and from neutrons that interact with the formations and the materials in the wellbore. Such neutrons may cause events in the radiation detectors that are not related to the properties of the formations desired to be evaluated.
Certain materials for scintillation detectors, for example, those described in the above referenced '403 patent, have physical characteristics such as being hygroscopic and being susceptible to damage by mechanical shock and vibration that make it desirable to mount the scintillation crystal to reduce the effects thereon of moisture, shock and vibration. It is also desirable to provide such mounting with suitable radiation shielding properties so that the radiation detector is primarily sensitive to radiation events of interest, while being relatively insensitive to other radiation events.